An aspect of optical information recording media is a phase-change information recording medium in which information is recorded, erased or rewritten by an optical system using laser beam. Examples of currently commercialized phase-change information recording medium include Blu-ray Disc media. More specifically, BD-RE media having two layers on one side (storage capacity of 50 GB, transfer rate of 36 Mbps (1× speed)) are commercialized as large capacity media capable of recording digital high-vision images.
The BD-RE media having two layers on one side includes two information layers, for example, a full reflection information layer L0 located far from the laser beam incident side, and a semi-transmission information layer L1 located on the laser beam incident side. The semi-transmission information layer is configured, for example, with a transmittance adjusting film, a reflective film, a dielectric film on the reflective film side, a first interface film, a recording film, a second interface film and a dielectric film on the incident side that are disposed in this order on a surface of an intermediate separation layer.
The transmittance adjusting film has a function of adjusting the transmittance of the information layer. Formation of this film using a material having a large refractive index enables an increase in transmittance of the information layer, and also enables the transmittance of a recording film in a crystal phase and the transmittance of a recording film in an amorphous phase to become closer to each other (see Patent Literature 1). In double layer BD-RE media, TiO2 has been used as the material of the transmittance adjusting film.
In order to achieve further increases in record contents for the future, promotions for PC applications, resource savings taking environment aspect into consideration and space savings, it is required for BD-RE media to increase capacity per disc. Examples of the method of increasing the capacity per disc include further increase in information layers to be laminated (multilayering) and further increase in storage capacity per information layer (high density recording). For the purpose of multilayering of the information layer, it is necessary to increase the transmittance of the semi-transmission information layers that are located nearer to the laser beam incident side, so as to read out information (in a high S/N ratio) from the information layer that is located farthest from the laser beam incident side.
To this end, it is necessary that a transmittance adjusting film is formed of a material that shows no absorption and has higher refractive index. Examples of typical material include oxides of Bi and Ti disclosed in Patent Literature 2. This material has a refractive index of more than 2.7 at a wavelength of 405 nm, and can further improve the transmittance of an information layer as compared with a conventional material. Accordingly, it is considered that this material is a material that will be widely applied in a multi-layered information recording medium in the future.
A recording film is formed of a phase change material that causes reversible change between crystalline and amorphous phases. Such a material is, for example, a material that is composed of three elements Te, Bi and Ge, and has a composition located on a line extending between Ge50Te50 and Sb40Te60 in a ternary diagram (see Patent Literature 3); a material that has a composition located on a line extending between Ge50Te50 and Bi40Te60 in a ternary diagram, a part of Bi being replaced with Sb (see Patent Literature 4); or a material that contains Sb as a main component (in the vicinity of 70 atomic %), and has a composition in the vicinity of an eutectic point of SbTe as a base (see Patent Literature 5).
Generally, information is recorded by irradiating the recording film with high-power (recording-power) laser beam thereby to heat the recording film to a higher temperature than its melting point, resulting in melting of the irradiated region, and then cooling rapidly to form an amorphous phase. In contrast, information is erased by irradiating the recording film with lower-power (erasing-power) laser beam than that used for recording thereby to heat the recording film to a temperature that is higher than its crystallization temperature but is lower than its melting point, resulting in temperature raise of the recording film, and then cooling slowly the recording film to form a crystalline phase. These crystallized region and amorphous region thus formed differ in reflectance from each other, and thus information can be reproduced using the difference in reflectance. Accordingly, higher quality reproduced signals can be obtained as a difference in reflectance increases.
A dielectric film on the reflective film side and dielectric film on the incident side have a function of adjusting an optical distance (=refractive index×physical distance) of an information layer thereby to increase light absorption efficiency of the recording film and to increase a difference between a reflectance in a crystal phase and a reflectance in an amorphous phase, and thus increasing the signal amplitude. These dielectric films also have a function of protecting a recording film from moisture. Examples of the materials of these dielectric films include a mixture of 80 mol % ZnS and 20 mol % SiO2 (hereinafter referred to as (ZnS)80(SiO2)20) (see, for example, Patent Literature 6 and Patent Literature 7). This material is an amorphous material, and has characteristics such as low thermal conductivity, high refractive index and high transparency. The material also shows high film formation rate during film formation, and is also excellent in mechanical characteristics and moisture resistance. Because of its excellent characteristics, (ZnS)80(SiO2)20 has been put into practical use as a material that is remarkably suited for forming a dielectric film.